The electrolytic reduction of chloropyridines with cathodes other than silver and the use of silver cathodes for electrolytic reductions of other types of compounds are known. The electrochemical activities of different varieties of silver electrodes have been the subject of several academic studies.
U.S. Pat. No. 3,694,332 discloses the use of a lead or mercury cathode for the electrolytic reduction of pentachloropyridine to symmetrical (2,3,5,6-) tetrachloro pyridine. The reaction is carried out in a solution of a neutral or acidic salt and water in an organic solvent. Some over-reduction of the tetrachloropyridine to an unidentified trichloro pyridine is indicated to occur if the reduction is continued until complete conversion of the penatachloropyridine is attained. However, environmental considerations now weigh against the use of mercury and lead in commercial processes and lead cathodes are not highly efficient for the reduction. Furthermore, efforts to extend the method of the patent to the preparation of products comprising substantial proportions of any trichloropyridines have been unsuccessful, not only with lead and mercury cathodes but also with other cathodes designated in the patent as suitable. (Several of the latter cathodes have hydrogen overvoltages as low or lower than silver.)
U.S. Pat. No. 3,010,966 discloses the use of a silver sheet as a cathode in the electrolytic cleavage of bis(4-hydroxymethyl-5-hydroxy-6-methyl-pyridyl-(3)-methyl)-disulfide dihydrochloride-hydrate to 3-mercaptomethyl-4-hydroxymethyl-5-hydroxy-6-methylpyridine. The reduction is carried out in 1:2 c.HCl/H.sub.2 O.
Current efficiencies for the electrolytic reduction of 2-butyne-1,4-diol, at silver cathodes of several different types were examined by Kato and Sakuma; J. Electrochem. Soc. Japan, Overseas Ed. 27, 236-9 (1959); C.A. 58 12,171e. White silver and bright silver cathodes gave poor efficiencies but compressed powdered silver cathodes had efficiencies of about 80%.
A correlation between hydrogen overvoltage and ease of reduction of several different substrates for different cathode metals was attempted by A. T. Petrenko; Electrokhimiya, 3(2), 252-5 (1957; in Russian) C.A. 66 121,518w. In the case of silver and copper, a parallelism between hydrogen overvoltage and effectiveness for the reduction of glyceraldehyde was not found (in contrast to a "perfect parallelism" for tin, lead and cadmium). The abstract does not indicate that data was obtained for reduction of chlorocarbons.
The effectiveness, for oxygen reduction, of several porous silver electrodes, prepared by leaching of pressed mixtures of silver powder and ammonium bicarbonate, has been examined by Burshtein et al.; Topl. Elem., 1968, 306-22 (in Russian), C.A. 73, 94,136f. The use of such porous silver electrodes--activated by inclusion of 2% of magnesium in a surface layer of the silver (on a carbonyl nickel layer)--for reduction of oxygen in 7 N KOH (at 90.degree. C.) was also investigated by the author; Electrokhimiya, 1970, 6(7), 939-48 (in Russian), C.A. 73 94,035x.
Utilization of frequent polarity reversals in the electrolytic deposition of trace amounts of noble metals (such as silver) in or below the surfaces of substrate metals (such as nickel or tantalum) is disclosed in U.S. Pat. No. 3,645,860, which is directed to the preparation of electrocatalysts. The noble metal source solutions are extremely dilute and the amount of the incorporated metal is far less than that which would be equivalent to even a monatomic layer.
Periodic polarity reversals of substantial relative duration are resorted to in the process for electroplating silver (from basic cyanide solution) onto substrate metals, disclosed in U.S. Pat. No. 2,678,909. In this case, the result of the polarity reversals is said to be smoothing (i.e., a decrease in surface area) of the plated metal.
The activation of metals, for use as electrocatalysts, be repeated polarity reversals is generally disclosed in Modern Electrochemistry; Bockris and Reddy; Vol. 2, page 1170; Plenum Press, N.Y., 4th Ed., 1973.
No art more pertinent to the present invention than the foregoing is known of and it thus does not appear that silver electrodes (cathodes) of the character required for the efficient electrolytic reduction of 2,3,5,6-tetrachloropyridine to 2,3,5-trichloropyridine are disclosed or made obvious by the known art. The latter compound is a convenient and economic intermediate for the preparation of 3,5-dichloropyridyl-2-oxy-phenoxy-substituted carboxylic acids (see U.S. Pat. No. 4,140,520, for example)--which are highly active herbicides. Pentachloropyridine is now a readily prepared starting material, so it is apparent that an efficient method of producing 2,3,5-trichloropyridine from pentachloropyridine is to be desired.